Metal to insulator transition for conducting polymers in plasmonic nanogaps

Conjugated polymers are promising material candidates for many future applications in flexible displays, organic circuits, and sensors. Their performance is strongly affected by their structural conformation including both electrical and optical anisotropy. Particularly for thin layers or close to crucial interfaces, there are few methods to track their organization and functional behaviors. Here we present a platform based on plasmonic nanogaps that can assess the chemical structure and orientation of conjugated polymers down to sub-10 nm thickness using light. We focus on a representative conjugated polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), of varying thickness (2-20 nm) while it undergoes redox in situ. This allows dynamic switching of the plasmonic gap spacer through a metal-insulator transition. Both dark-field (DF) and surface-enhanced Raman scattering (SERS) spectra track the optical anisotropy and orientation of polymer chains close to a metallic interface. Moreover, we demonstrate how this influences both optical and redox switching for nanothick PEDOT devices.

The circuit model (main text Eqn.1) can predict the general tuning direction of the coupled mode wavelength in response to changes in gap material permittivity and gap size.However, it does not account for nanoparticle facet size and shape, which can also influence the optical coupled modes in NPoMs.A more precise (and recent) model exploits the Quasi-Normal Mode (QNM) decomposition of NPoMs (see ref [1], and https://www.np.phy.cam.ac.uk/npomcalculator which is an online NPoM mode calculator).This method considers facet shape (circular, triangular, square) and facet fraction ( = /, where  is facet diameter, and  is NP diameter).The circuit model, the QNM decomposition method (circular facet, = 0.35), and experimental data from Fig. 1e are compared in Fig. S2b below.These simplistic models miss another point: without external potential, PEDOT is partially oxidised, giving a strong resonance between 550-750 nm in the real part of its permittivity (Fig. S5).These features explain why such models cannot exactly predict the experimental data.Extracted peak intensity vs applied potential of (i) symmetric (Cα=Cβ) for PEDOT 0 at 1433 cm -1 and PEDOT 2+ at 1456 cm -1 , and (ii) asymmetric (Cα=Cβ) for PEDOT 0 at 1513 cm -1 and PEDOT 2+ at 1540 cm -1 .(c) Fractional ratio of the two components, defined as   /(  +   ) at each V. Labels 'ox' and 're' in legend represent PEDOT oxidation (0.6→-0.6 V, P 2+ →P 0 ) and reduction (-0.6→0.6 V, P 0 →P 2+ ) processes.

Fig. S1 :
Fig. S1: PEDOT thickness measured by DLS Fig. S2: Field distribution for typical resonance wavelength in eNPoMs Fig. S3: Optical switching of Au@PEDOT eNPoM Fig. S4: Drude model of used for metallic state Fig. S5: Anisotropy in PEDOT permittivity at different charge states Fig. S6: Calculated DF scattering spectra of NPoM with 15 nm thick isotropic PEDOT disc Fig. S7: Calculated DF scattering spectra of eNPoM with 15 nm thick isotropic PEDOT shell Fig. S8: Calculated DF scattering spectra of NPoM with anisotropic PEDOT disc Fig. S9: SERS spectra for eNPoMs at different shell thickness Fig. S10: Comparison of time-scan SERS spectra for 2 nm and 13 nm gap PEDOT shell Fig. S11: In situ cyclic-voltammetry SERS time-scan spectra of eNPoMs Fig. S12: Raman evolution of eNPoM with 20 nm shell during redox Fig. S13: Analysis of SERS dynamics of peak of interets ( 1 - 6 ) during redoxTable S1: Assignments of characteristic Raman bands for PEDOT

Fig. S1 |
Fig. S1 | PEDOT thickness measured by DLS.(a) Typical nanoparticle size distribution measured using DLS of (citrate capped) gold nanoparticle before and after polymer growth with 10 mM monomer.(b) Calculated PEDOT shell thickness as a function of monomer concentration.Error bar: Standard deviation of polymer shell.

Fig. S2 |
Fig. S2 | (a) Field distribution for typical coupled resonance wavelength in eNPoMs, 80 nm diameter, 1.5 nm gap size.(b) NPoM coupled mode plasmon  vs PEDOT shell thickness  (in air) compared with predictions from the circuit model and QNM model (circular facet, f=0.35).Error bar shows width of  histogram.

Fig. S4 |
Fig. S4 | Drude model used for metallic state.(a) Real and (b) imaginary parts of the permittivity used for the metallic phase of the gap in simulation.Points show permittivity of gold measured from Johnson and Christy 2 (copyright Americal Physical Society).Parameters of Drude fitting are adapted from Ref [3].

Fig. S8 |
Fig. S8 | Calculated DF scattering spectra of NPoM with (a) 2 nm and (b) 15 nm thick anisotropic PEDOT disc (radius: 15 nm) in the gap for reduced (purple, P 0 ) and oxidised (orange, P 2+ ) states.(i-iv) show different combinations of two different illumination conditions and PEDOT permittivity seen by the optical field.

Fig
Fig. S10 | (a) Example of unstable picocavity SERS spectra collected from two different eNPoMs with 2 nm gap.Laser power 6 μW.Integration time: 1 s.(b) Example of a stable nanocavity SERS spectra, collected from an eNPoM with 13 nm gap.Laser power: 3 μW.Integration time: 1 s.